Jet Calibration

Of importance in a top quark mass measurement is the understanding of the jet en-ergy scale (JES). The JES is the response difference between the original objects and the energy determined in the detector. This translation in energy returns the energy of the measured object back to its particle level. Jet calibration is performed to account for either dead material of the detector, or the different responses in the hadronic and electromagnetic calorimeter. A calibration referred to as EM+JES is performed. The calibration is determined from a scheme at the electromagnetic scale with jet energy scale calibration [108, 109]. The scale uses only the electromagnetic corrections of cells and corrects the jets based on their pT and η. The material and difference between hadronic to electromagnetic response is also taken into account. The different components of the JES are:

Calorimeter Response

The calorimeter response is a result of the measured response of the individual particles contained within a jet. Each individual particle is compared to the deposition in the calorimeter cell. The response is based on η and p_T of the jet. This has been measured in MC and verified in 2010 data. The uncertainty on the response is found to be between 1.5 % and 4 %. For high pT jets (pT > 100 GeV/c), this term dominates the total JES.

Underlying Event

The underlying event in the default pythia MC is changed to the PERUGIA2010 modeling, which has been previously derived at the Tevatron. The differences in JES of the underlying event to the nominal underlying event used in pythia are taken as the uncertainties.

η-Intercalibration

An η-intercalibration is performed over the completeη range of the detector to obtain a uniform response. The response for different detector regions is not uniform due to effects such as dead material and differing amounts of detector materials. To measure the uncertainty in calorimeter response, dijet events in which one of the two jets is found in the central η range are studied. The central η range is well understood and thus the second jet acts as a probe of the not-well understood detector range. Comparisons of data to MC response allow the determination of an uncertainty of the JES. This component dominates the JES uncertainty at low pT regions (pT < 50 GeV) and forward detector regions.

Parton Shower Model

The defaultpythiaMC is exchanged with a differing shower model. The shower model used as a variation is alpgen with herwig and jimmy models. The difference is taken as the uncertainty.

4. Event and Object Reconstruction

Close-by Jets

Events at the high luminosity of the LHC contain many jets, some of which are overlap-ping. Close-by jets may result in the degradation of the calorimeter response to a nearby jet. As a result, an additional JES uncertainty term is added to account for this effect.

This effect was studied in QCD MC events and compared to data. The total effect is found to be less than 3 % [110].

Noise Thresholds

In the creation of jets through topological cluster algorithms, a noise over threshold is used on calorimeter cells. Variations in the noise may effect the jet shape, as it has an effect on the topological clustering. The response is checked in data and MC and the differences in the response are taken as systematic. The effect is limited to low pT jets and found to be of minimal effect on the total JES.

Flavour Composition

Due to the differences in jet shape, an additional uncertainty is the result of the quark-gluon composition of the sample. To account for the differences in jet flavour, measure-ments are made of the response in the calorimeter from gluon or quark dominated samples.

For background samples, a 50:50 composition sample is used and for signal samples a more quark dominated sample is used for the JES.

Pileup

Due to the increase in luminosity in data collected in 2011, the effect of pileup on the MC is not properly modeled. An additional uncertainty is added to the MC to account for the underestimation of pileup on the calorimeter response. The pileup component has an additional uncertainty of up to 5 to 7 % depending on the detector region, within|η|<2.5.

The pileup component of the JES is found in Table 4.1. It is added in quadrature to the other components.

Uncertainty [%] |η|<2.1 |η|>2.1 pT <50 GeV 5 % 7 % 50<pT <100 GeV 2 % 3 % pT >100 GeV 0 % 0 %

Table 4.1.: JES uncertainty component due to pileup. The additional uncertainty is added in quadrature to the other components. The largest uncertainties are found at forward jet regions and low pT jets.

To quantify the calibration for jets used at the EM scale, the jet response is plotted along with the total JES in Figure 4.3. The JES is split into the various components and the combination of the uncorrelated terms are added together and shown as a function of the jet p_T.

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4.1. Object Definition

Figure 4.3.: (Top): Jet Response at EM Scale for different regions of the detector. Dif-ferent energies are highlighted. The transition regions show a large drop in response and are thus excluded from reconstructing objects. (Bottom left):

JES breakdown in the barrel region from several MC and detector com-ponents. The largest JES component is the calorimeter response, which at high pT, almost completely accounts for the total uncertainty. (Bottom right): JES breakdown in the forward region, |η| > 2.1. Figures taken from [111].

The jet response at EM scale is plotted for different η regions of the detector. Theseη segments are also used for the JES determination. The response is quite similar through all ranges of the detector for a given energy, except in the transition regions such as 1.37<|η|<1.52. This region will also be excluded for calorimeter reconstructed objects.

The JES uncertainty is shown against the jet p_T. The different contributions to the JES are also shown. It can be seen that the largest uncertainty, especially at high pT, is due to the calorimeter response. At low p_T, the JES is dominated by the η-intercalibration.

The dependence on pile-up is not shown in the total JES calculation, however it does also account for a significant portion of the JES on top of the plotted values. The additional JES due to pile-up is up to 7 % in forward regions (|η| > 2.1) and 5 % in the central region. The total JES uncertainty ranges from 2.5 % to 8 % in total. In addition to the JES, an additional bJES of up to 2.5 % is added in quadrature to true b-jets due to the fragmentation of b-hadrons.

4. Event and Object Reconstruction